Efficient wastewater treatment is necessary to maintain the chemical, physical, and biological integrity of the environment. The Clean Water Act has established standards for discharges from all municipal wastewater treatment plants discharging to surface water. The treatment of wastewater involves the removal of dense particles, debris, and organic solids. Additional treatment can also be employed to remove colloidal and soluble organic matter. In more advanced wastewater treatment plants, steps may be taken to remove nutrients, defined as compounds containing nitrogen and phosphorus, and the effluent may be filtered.
Nutrient removal is becoming more important as the effects of excessive nutrients on waterways is better understood. High nitrogen and phosphorus concentrations have been identified as sources of excessive phytoplankton and algae growth, which in turn leads to the eutrophication, or hypertrophication, of waterways. Excessive phytoplankton and algae growth often leads to blooms, which may lead to hypoxia and anoxia and the death of native fish and aquatic species.
To remove excess nutrients from wastewater, processes have been developed to remove nitrogen containing compounds from the wastewater and remove phosphorus from the wastewater. State of the art wastewater treatment facilities create different environmental conditions in selectors, defined as aerobic (i.e. presence of free dissolved oxygen in the wastewater), anoxic (i.e., absence of free dissolved oxygen but presence of oxidized forms of nitrogen such as nitrate and nitrite), anaerobic (i.e., absence of free dissolved oxygen and oxidized forms of nitrogen) and settling selectors, to accomplish nitrogen and phosphorus removal. These selectors are often maintained in separate tanks or vessels that are configured in a specific manner to accomplish contaminant removal objectives. One such process currently in use is the Bardenpho® nutrient removal system. The Bardenpho® process uses suspended microbial mass, called activated sludge, transferred through a series of selectors to remove nitrogen and phosphorus in addition to the dissolved organic (carbonaceous or carbon-containing) material. A five stage Bardenpho® comprises five selectors: an anaerobic selector, a first anoxic selector, an aerobic selector, a second anoxic selector, and a re-aeration selector.
Activated sludge in the anaerobic selector is stressed in the presence of carbonaceous materials, resulting in phosphorus release from the activated sludge that allows even greater levels of phosphorus to be removed from the wastewater in subsequent anoxic and aerobic selectors via a process called enhanced biological phosphorus removal (EBPR). Externally added carbonaceous material (i.e., a carbon source) can be added in this anaerobic selector to improve the EBPR process by generating a greater phosphorus release in the selector, resulting in higher EBPR activity and greater phosphorus uptake in subsequent anoxic and aerobic selectors. In the first anoxic selector, oxidized forms of nitrogen (e.g., nitrite and nitrate) are reduced to gaseous nitrogen in the absence of free dissolved oxygen typically utilizing the carbonaceous material in the wastewater. This process of reducing nitrate and nitrite to gaseous nitrogen is called denitrification. A carbon source can be added in this selector to achieve more complete removal of nitrate and nitrite. Uptake of phosphorus also occurs in the anoxic selector by denitrifying phosphorus accumulating organisms (DPAOs). Oxygen is then added in the aerobic selector to oxidize ammonia and other reduced forms of nitrogen (e.g., organic nitrogen) as well as the remaining dissolved carbonaceous material, converting the reduced forms of nitrogen to nitrite and nitrate, a large portion of which is recycled to the first anoxic selector. In an EBPR process the majority of the biologically available phosphorus is taken up in the aerobic zone by phosphorus accumulating organisms (PAOs). In the second anoxic selector, the remaining nitrite and nitrate are denitrified to gaseous nitrogen in the absence of oxygen utilizing a carbon source since the carbonaceous material in the wastewater is typically fully consumed (i.e., oxidized) in the nitrification (i.e., aerobic) selector. Conditions in this second anoxic selector can also be made to be similar to those in the anaerobic selector using the external carbon source as the driver to create those conditions, allowing for additional phosphorus removal in the re-aeration selector. Following re-aeration, a metal compound can be added to precipitate any remaining phosphorus in a chemical phosphorus removal step in a settling selector such as a clarifier.
In place of activated sludge, other systems have been developed which also utilize an external carbon source to denitrify the wastewater, utilize a carbon source to increase phosphorus removal by the EBPR process, and dose a metal compound for chemical phosphorus removal. For example, a fixed film denitrification filter can be utilized as a denitrification process. A metal compound can be added to a tertiary filter for chemical phosphorus removal. Additionally, EBPR processes can occur in side stream selectors and can contain a fixed film medium or a separate activated sludge process from the main stream selector group.
In some instances, environmental conditions can be modified in single tank systems such as a sequencing batch reactor. In a sequencing batch reactor the same vessel is used for aerobic, anoxic, anaerobic and settling steps. The wastewater is treated in a single batch and the environmental conditions are modified throughout the batch to accomplish nitrogen and phosphorus removal.
In some instances more than one condition can exist in a single selector. For example an anaerobic selector may be partially aerobic or anoxic at the influent of the selector and then transition to an anaerobic condition after free and chemically bound oxygen are reduced. In another example, a condition may be anoxic at the influent of a selector and may become anaerobic after nitrate and nitrite are removed via denitrification.
In many common configurations, such as activated sludge processes, although the selectors are often separate reactors, the activated sludge is common throughout the system. As the activated sludge passes through the different selectors, certain groups of bacteria perform different functions. For example, in an aerobic selector, nitrifying bacteria convert ammonium to nitrate and PAO's take up phosphorus. In anoxic selectors, a group of microorganisms called heterotrophs convert nitrate to nitrogen gas and DPAO's take up phosphorus. In anaerobic selectors, PAOs and DPAOs store energy in the form of polyhydroxylalkanoates (PHA) and other intracellular storage compounds and release phosphorus. Certain groups of bacteria are therefore more active in some selectors and less active (or not active at all) in others. Because the microbial population is the same throughout the system, the nitrogen and phosphorus treatment in one selector has a direct impact on the performance of another selector.
It is common in the art to add an external carbon source such as methanol, ethanol, other alcohols, acetate, glycerin, carbohydrates and other organic compounds such as hydrocarbons to stimulate denitrification, particularly in selectors in which the organic content in the wastewater has been consumed or is no longer readily biodegradable. The addition of carbon sources serves as an electron donor that the denitrifying bacteria utilize to convert nitrate to nitrogen gas. It is also common in the art to use a control strategy to appropriately dose the carbon source to achieve denitrification performance.
More recently, external carbon sources have been added for EBPR. The types of carbon sources that support EBPR are more restricted than those for denitrification. For example, methanol, the most widely used carbon source for denitrification is not known to support EBPR. Carbon sources that support EBPR are, or need to be, converted to volatile fatty acids or other precursors to PHA which are then stored intracellularly as PHA's and other storage products under anaerobic conditions concomitant with phosphorus release from intracellular polyphosphate. Commonly used carbon sources for EBPR are acetate, propionate, carbohydrates, glycerol, and the like. As the PAO's and DPAO's move from the anaerobic selector to other selectors, the stored PHA is oxidized using nitrate, nitrite or oxygen which results in energy production and uptake of phosphorus as intracellular polyphosphate. Therefore, in systems utilizing EBPR for phosphorus removal, the biochemistry occurring in the anaerobic zone has a direct impact on carbon utilization, nutrient removal and deoxygenation in subsequent selectors. Denitrification and EBPR processes are tightly linked in the treatment facility, even though the two processes may be occurring in separate selectors. A central control system that manages the carbon, nitrogen and phosphorus removal processes in multiple selectors within the treatment process optimizes overall system performance.
Chemical phosphorus removal is typically accomplished by adding metal salts such as calcium, aluminum and iron based compounds. Two examples are aluminum sulfate (alum) and ferric chloride. Other non-limiting examples include poly aluminum chloride, sodium aluminate, calcium hydroxide, ferrous chloride, ferric sulfate, ferrous sulfate and magnesium chloride. These metal salts can be added in different selectors but are most often added to selectors called settling tanks or clarifiers. When added to water, aluminum and iron salts take the form of metal hydroxides that bond with soluble phosphoate resulting in a metal-phosphate complex that precipitates out of the water. Metals form complexes with phosphorus, resulting in phosphorus precipitation. Rare earth metals and metal salts such as cerium chloride can also be used to remove phosphorus from water by forming chemical bonds with certain phosphorus species that result in phosphorus precipitation. Other metal compounds can be used in place of metal salts, such as metal oxides, so long as the metal is available to react with the phosphorus when dosed to the water treatment system. In all cases, the precipitated phosphorus compounds are typically removed from the wastewater with the waste solids, using methods such as sedimentation or filtration. It is common in the art to use a control strategy to appropriately dose the metal compound to optimize chemical phosphorus removal.
Other factors also influence the concentration of nitrogen and phosphorus within the wastewater during treatment. For example, phosphate accumulating organisms (PAOs) use stored polyphosphate as an energy source and release phosphorus in the anaerobic stage. Other organisms known as glycogen accumulating organisms (GAOs) take up volatile fatty acids (VFAs) from the wastewater and store them internally as glycogen. Unlike with PAOs, when GAOs form glycogen, it does not result in a phosphorus release into the wastewater. The populations of PAOs and GAOs in the wastewater are affected (and can be controlled) by specific operating conditions, for example, the pH of the wastewater and the age of the sludge. By using one or more feed control algorithms tied to process measurements, consumption of added carbonaceous material (i.e., an external carbon source) by other microbial processes that consume carbon in excess of the amount required by the target processes can be minimized or eliminated (e.g., external carbon consumption by GAOs and/or PAOs over the required amount needed by denitrifying microorganisms).
The demands on wastewater treatment plants are known to vary dramatically. Seasonal and usage demands vary greatly throughout the year. With these changes, it is often difficult to maintain constant effluent concentrations and comply with discharge requirements. It is therefore desirable to develop a wastewater treatment system that can efficiently adjust to changing demands.
Wastewater treatment plants are often operated using constant external carbon source feed systems. With constant feed systems, the addition rate of external carbon source and/or metal compound is kept constant. This feed system can have many drawbacks. Constant feed systems are unable to adapt to the constantly changing needs of the wastewater treatment system. Therefore, it is often necessary to overcompensate with the amount of external carbon source or metal compound fed to the system. This leads to wasted resources and extra strain on the wastewater treatment system. During times of peak nutrient influx, the constant feed system may not be able to adequately remove nitrogen and phosphorus from the wastewater.
More advanced wastewater treatment systems may use a nitrogen sensor to detect the amount of nitrogen within the incoming wastewater. This system may adjust the amount of external carbon source based on the detected incoming nitrogen level. However, these systems may be slow to react to rapidly changing influx conditions and/or may not adequately provide for phosphorus removal.
Computer controlled processes and systems for the automatic dose control of wastewater treatment chemicals and the denitrification of wastewater streams are known in the art, as exemplified by U.S. Pat. Nos. 6,129,104; 7,153,429; and 8,034,243 and U.S. Pub. No. 2012/0211417 (the entire disclosures of each of which are incorporated herein by reference in their entireties for all purposes). However, there remains a need in the art of wastewater treatment systems for more advanced controls. Such advanced controls may improve the removal of nitrogen and phosphorus from wastewater, improve the reaction time for changing conditions, improve reliability of final effluent discharge compliance, and/or reduce the amount of external carbon source and metal compound needed by the system.